I consider the entire model engine intake assembly (before the crankshaft) to simply be a fuel/air metering device that has very little atomization capabilities. (And the more you try and chase atomization capabilities the more inconsistent it becomes in a changing stunt environment.)

Witness the fact that one can run an engine on a burst just by priming it with straight fuel, no droplets, no atomization, just a small solid volume of fuel as most of the atomization occurs much later in the exit of the transfer ports - e.g. small ports give high speed and far better flow and atomization just like miniature low pressure fuel injectors.

Thus one should look at how well any stunt venturi system will give a consistent metering of sheared droplets and not bother even considering how fine the resultant spray is going to be.

I consider the entire model engine intake assembly (before the crankshaft) to simply be a fuel/air metering device that has very little atomization capabilities. (And the more you try and chase atomization capabilities the more inconsistent it becomes in a changing stunt environment.)

Witness the fact that one can run an engine on a burst just by priming it with straight fuel, no droplets, no atomization, just a small solid volume of fuel as most of the atomization occurs much later in the exit of the transfer ports - e.g. small ports give high speed and far better flow and atomization just like miniature low pressure fuel injectors.

Thus one should look at how well any stunt venturi system will give a consistent metering of sheared droplets and not bother even considering how fine the resultant spray is going to be.

    Atomization is absolutely critical to reliable stunt runs. David and I have experimented a lot with both the fuel feed and the air intake over the past 10 or so years, following on to Frank Williams' experiments with spigot venturis. Contrary to the laminar flow or pressure recovery theories, the most reliable and consistent runs come from venturis with the most turbulent flow, and even something like the panty hose diffuser/filter can make a marked difference in the run quality. The first example was the astonishing improvement in the inside/outside speed variation by switching to a spigot venturi just exactly like Frank's article on the topic. If we hadn't solved that I would still be running 40VFs.

    I am pretty convinced that despite the fact that it might flow better on a bench, the very last thing you want is anything like streamlined flow and you certainly do not want the charge flow attaching to the walls and just dribbling into the engine. Sergei Belko's "top intake" engine is intriguing because it allows you to use the entire nose of the airplane as an airbox or plenum. I think that's why the original ST sprinkler venturi worked better than a lot of the later "flush inlet" types, even if you drill out the seat where the sprinkler holes are. There are a few sharp edges right upstream of the holes that certainly break up any notions of laminar flow.

    There's a few long threads on this topic in the archives on SSW, but we have gotten smarter since then. And I am sure our mavens for rigor will be down on me for the rampant speculation and anecdotal evidence shown here.

     The other thing that appears to make a huge difference is how the fuel supply path works, and the viscosity and flow characteristics of the fuel itself. The flow rate is pretty slow ON AVERAGE, but it's not constant so the the peak flow rates can apparently be pretty high. This we learned from both Brian Eather's observations on fuel viscosity and the early problems with 4-strokes on overheads. One of the keys, with a remarkable improvement, was switching (at Brad Walker's suggestion at the 2002 NATs, probably originally from Bill Wilson and RC guys) to a clunk tank. That led to looking at what was going on with fuel flow.

    Some of what found on the fuel path is still semi-secret, sorry about that, because I didn't invent it and it has not been published.  I am sure that there will be 5 people claiming that they know what we are doing, they originally invented it,  and we're stupid, but that's the way it goes sometimes. For the most part, reduce the flow restriction to the extent possible even if it seems pointless and works OK the way it is now. But don't make the spigot itself too big, anything downstream of the needle needs to be small enough to stay wetted all the way across the tube. If it's too big after the fuel is metered, it's possible to get the fuel flowing down one side and air coming in the other side. That's probably OK if it stayed that way all the time and never changed, but it will.

     Brett

Brett,

Thank you. Other than pantyhose, I would be interested in other ways to make the flo turbulent. I don't like the panties as the venturi is quite well hidden behind the engine, And I use a syringe with needle for priming.
Should I just make steps inside the venturi before spigot? Or should the inside diameter increase more aggressively right after the spigot/fuel hole. The last option is the standart in team racing engines which are also very sensitive to these issues. L

What effect does density altitude have on electrics?

Charles 

 I use a corner rounding tool in the tail stock to obtain a round entry on the venturi. The tool could be mounted in the tool post allowing feed in two directions to blend into the diameter. A 15 degree reamer chucked up in the tail stock can be used to obtain taper. Finally, a tip from John McCollum, an end mill mounted in a tool post makes a very smooth cutting boring bar. I hope I am not repeating something already covered.
Phil

Another semi on topic tale of the value of knowing what you are working with and why different items that are the "same"(acetal homopolymer) may not work the same.

My brother worked at 3M in the tapes division.  They made lots of electrical insulating tape.  He mainly did troubleshooting, and got called in because the rolls of tape were blocking(the adhesive was sticking to the next layer) and pulling the adhesive off when the roll was unwound.  Not so good.  They spent 2 months going over their processing and materials, the line conditions, everything.  It turned out that the manufacturer of an antiblocking additive that they put into the plastic tape had changed suppliers for one of the ingredients.  The new ingredient was chemically the same, but had a solvent in it from a different supplier.  The solvent ended up in the ppm range in the tape, but the change was enough to keep the main antiblocking agent from getting properly lined up on the surface of the tape to do its job of keeping the adhesive from sticking too hard.

So I'm pretty sure that if a machinist says Delrin works better than generic acetal he's speaking the truth.  There are about 50 variables between polyoxymethylene and finished, extruded rod.  All of the polymerization variables are critical, and hard to control, and can have major effects on the finished product.  Things like friction, glass temperature, deformation at various temperatures, how well the internal lubricants are dispersed and dissolved, nearly ad infinitum.